The invention relates to a method of estimating the time of arrival of a radio signal and to an apparatus operating in accordance with the method, and has particular, but not exclusive, application to location systems for mobile radio devices.
Location systems for mobile radio have a variety of applications, for example for use in conjunction with emergency calls made from mobile telephones, as many people making emergency calls are unaware of their precise location, making it difficult for the emergency services to reach them. A variety of technologies have evolved for determining the location of a mobile radio. A review of location technology is contained in xe2x80x9cAn Overview of the Challenges and Progress in Meeting the E-911 Requirement for Location Servicexe2x80x9d by J. H. Reed et al, published in the IEEE Communications Magazine, Apr. 1998, pp. 30-37.
The location of a mobile radio may be determined by calculating its distance from three or more radio base stations situated at known locations. The location of the mobile radio will be at the intersection of circles (or spheres in three dimensions) about each base station, each circle having a radius corresponding to the distance of the mobile radio from that base station. Such distances may be readily determined from the propagation delay of a radio signal travelling between the mobile radio transmitter and base station receiver, assuming that radio signals propagate at the constant speed of light (3xc3x97108 m/s). For example, a propagation delay of 10 xcexcs will correspond to a distance of 3 km. The propagation delay can be determined by combining what is commonly referred to as a TOA (Time Of Arrival) measurement with knowledge of the time at which the radio signal was emitted from the transmitter.
A more practical method of determining propagation delay, in which knowledge of the emission time is not required, is to measure the difference in time of a signal emitted by the mobile radio transmitter arriving at each base station receiver. Instead of the circular location trajectory about each base station which results from an absolute TOA measurement, for each pair of base stations there is calculated a hyperbolic trajectory corresponding to the difference in distance of the mobile from each base station in the pair, and the location of the mobile radio will be at the intersection of these hyperbolae. Measurement of the difference in time of arrival is commonly referred to as a TDOA (Time Difference Of Arrival) measurement.
Methods of calculating location using TOA and TDOA measurements are discussed in xe2x80x9cPosition Location Using Wireless Communications on Highways of the Futurexe2x80x9d by T. S. Rappaport et al, published in the IEEE Communications Magazine, Oct. 1996, pp. 33-41.
Whether TOA or TDOA measurements are used, the accuracy of the location calculation is very dependent on the accuracy of the time of arrival measurements. In the present specification and claims the term xe2x80x9ctime of arrivalxe2x80x9d is used to encompass both TOA and TDOA.
Time of arrival can be measured by using the correlation properties of a pseudo-noise (PN) sequence, transmitted from a transmitter to a receiver. The correlation of the received signal with a replica of the transmitted signal will produce a peak in the correlation function when the two are synchronised. The peak can be detected even in the presence of severe distortion caused by multipath propagation and noise. Therefore, the problem of measuring time of arrival is equivalent to the accurate detection of the position of the peak in the correlation function between the distorted, received signal and a replica of the transmitted signal. It is known that correlation of the received signal with the replica can be performed in either the time domain or in the frequency domain, for example see T. S. Rappaport et al referenced above.
In emergency situations, the speed and accuracy of location calculation can be critical. The speed and accuracy of the location calculation is dependent on the speed and accuracy of the time of arrival measurement, which, in turn, depends on the processing power available and the algorithms used. In order to address a mass market, equipment cost should be low, so a low processing power requirement is important.
An object of the invention is to provide improvements in location systems for mobile radio by providing improvements in the time of arrival estimation.
According to one aspect of the invention there is provided a system for determining the location of a mobile radio station relative to radio base stations, comprising a plurality of radio base stations, each equipped to transmit a signal; and a mobile radio station equipped to receive the signal transmitted by each radio base station and having means for estimating the time of arrival of each received signal and means for computing the location of the mobile radio station from the estimated time of arrival of each received signal, wherein the means for estimating the time of arrival further comprises a means for making a first estimate of time of arrival having a first resolution and a means for making a second estimate of time of arrival having a second resolution, the first estimate of time of arrival contributing to defining the scope of the means for making the second estimate of time of arrival, and the second resolution being more refined than the first resolution.
According to a second aspect of the invention there is provided a system for determining the location of a mobile radio station relative to radio base stations, comprising a mobile radio station equipped to transmit a signal, a plurality of radio base stations, each equipped to receive the signal transmitted by the mobile radio station and having means for estimating the time of arrival of the received signal, and means for computing the location of the mobile radio station from the estimated time of arrival of the signal received at each radio base station; wherein the means for estimating the time of arrival further comprises a means for making a first estimate of time of arrival having a first resolution and a means for making a second estimate of time of arrival having a second resolution, the first estimate of time of arrival contributing to defining the scope of the means for making the second estimate of time of arrival, and the second resolution being more refined than the first resolution.
According to a third aspect of the invention there is provided a method of determining the location of a mobile radio station relative to radio base stations, comprising transmitting a signal from a plurality of radio base stations; receiving at a mobile radio station the signal transmitted by each radio base station; estimating the time of arrival of each received signal; and computing from the time of arrival estimates the location of the mobile radio station; wherein the time of arrival estimation further comprises making a first stage of estimation having a first resolution and which yields a first estimate of time of arrival, and making a second stage of estimation having a second resolution and which yields a second estimate of time of arrival; the first estimate of time of arrival contributing to defining the scope of the second stage of estimation, and the second resolution being more refined than the first resolution.
According to a fourth aspect of the invention there is provided a method of determining the location of a mobile radio station relative to radio base stations, comprising transmitting a signal from a mobile radio station; receiving at a plurality of radio base stations the signal transmitted by the mobile radio station; estimating the time of arrival of each received signal; and computing from the time of arrival estimates the location of the mobile radio station; wherein the time of arrival estimation further comprises making a first stage of estimation having a first resolution and which yields a first estimate of time of arrival; and making a second stage of estimation having a second resolution and which yields a second estimate of time of arrival; the first estimate of time of arrival contributing to defining the scope of the second stage of estimation, and the second resolution being more refined than the first resolution.
According to a fifth aspect of the invention there is provided an apparatus for estimating the time of arrival of a radio signal transmitted between a fixed radio station and a mobile radio station, comprising first means for making a first estimate of time of arrival, the first means having a first resolution, and second means for making a second estimate of time of arrival, the second means having a second resolution, the first estimate of time of arrival contributing to defining the scope of operation of the second means, and the second resolution being more refined than the first resolution. In one embodiment of the invention a PN sequence is transmitted from a radio base station and received by a mobile receiver. In the receiver two stages of time of arrival estimation are used.
In the first stage of estimation, a frequency domain calculation is performed as follows. The Fast Fourier Transform (FFT) of the received signal is calculated and multiplied by the FFT of a replica of the transmitted signal. The inverse FFT of the resulting product is calculated, which yields the correlation function of the received signal and the transmitted signal. A distinct peak in the correlation function corresponds to a strong similarity between the received signal and the replica, thereby indicating the reception of the transmitted sequence, and the position of the peak indicates the time of arrival of the sequence within the received signal. There may, in addition, be minor peaks resulting from multipath propagation, but these are processed separately and therefore can be ignored in the context of this invention.
For this first stage of estimation a coarse sampling interval is used, for example in the range of a quarter to a sixteenth of the period of a chip of the transmitted sequence. This results in only a coarse estimate of the time of arrival with a resolution equal to the sample interval, but the processing is fast to perform and requires only a low processing power.
After the position of a distinct peak in the correlation function has been located in the first stage of estimation, the received signal is processed in a second stage of estimation.
In the second stage of estimation, a time domain calculation is performed as follows. The cross correlation function of the received signal and a replica of the transmitted signal is calculated by calculating the cross correlation coefficient for successive shifts of the replica in the region of the peak in the correlation function detected during the stage one estimation, the shift increment being smaller than the sample interval used in stage one, until a peak in the correlation function has been identified. By using a small shift increment the position of the peak can be determined more precisely than in the first stage of estimation. This peak indicates the time of arrival of the transmitted sequence at the mobile receiver.
The second stage of estimation results in a higher resolution estimate of time of arrival than stage one. In general, the stage two estimation is more processor intensive than stage one, but the second stage of estimation is confined to the region of the coarse estimate detected in the first stage of estimation. The combined result of stages one and two is an estimate of time of arrival which can be more accurate and/or faster and/or less processor intensive than a pure frequency domain estimation or a pure time domain estimation. The desired trade off between these benefits may be selected by the equipment designer.
PN sequences are also transmitted from additional radio base stations and the mobile receiver calculates the time of arrival of each sequence that it can detect. The sequences from different radio base stations can be distinguished by use of different sequences and/or different frequencies.
The estimated times of arrival of the sequences from each radio base station are then used by the mobile receiver for the calculation of receiver location using TOA or TDOA system formulae that are well known in the art. To enable the location of the receiver to be calculated, signals must be received from at least three radio base stations.
In a second embodiment of the invention a sequence is transmitted from a mobile radio and received by radio base stations. In this case the stage one and two estimation of times of arrival, and the subsequent calculation of mobile radio location, is performed within the fixed network rather than in the mobile radio. In order to calculate the location of the mobile radio, the signal transmitted by the mobile radio must be received by at least three radio base stations.
A benefit of the first embodiment, in which the mobile radio calculates its own location, is that mobile radios having location capability can be readily introduced with zero or little upgrading of the fixed network, and any number of mobile radios can provide location information to their users without creating a load on the network.
A benefit of the second embodiment, in which the calculation of the mobile radio location is performed in the fixed network, is that a mobile radio location service can be introduced with zero or little upgrading of the mobile radio.
Examples of applications where the invention may be beneficially applied are: handheld or vehicular positioning devices such as GPS (Global Positioning System) receivers; handheld or vehicular mobile radio communication devices, such as mobile telephones, with a positioning capability; and handheld or vehicular radio data communication devices, such as mobile internet terminals, with a positioning capability.